1. Field of the Invention
The present invention relates to a semiconductor component comprising a first layer consisting of semiconductor material as a substrate, a second layer running on this first layer, at least a first and a second intermediate layer made of the materials of the first and second layers running between the first and second layers, as well as an electroconductive contact that forms a connection to the first layer and originates at the second layer or runs through the second layer.
Further, the present invention relates to a method for producing a metal-semiconductor contact of a semiconductor component, such as a solar cell, comprising a first layer as a substrate made of semiconductor material as well as a second layer applied to it made of basic contact material consisting of or containing metal.
2. Description of Related Art
In the production of semiconductors, in particular in the production of solar cells, sintered metal contacts are used for the front and back surfaces of the cell for production costs reasons.
A silicon solar cell usually has a large-area aluminum layer on the back surface, which is exposed to a sintering process by means of thermal treatment during production of the solar cell, as a result of which the back surface of the solar cell is simultaneously passivated by means of a so-called back-surface field (BSF).
On sintering, the aluminum layer, which is in direct contact with the silicon substrate that should be designated as the first layer, is melted at the boundary layer between the aluminum layer and the silicon substrate and alloyed with the adjacent first layer. On cooling, the highly Al doped silicon layer solidifies epitactically on the back surface of the wafer facing the silicon, namely the substrate. Simultaneously, a silicon enriched Al layer solidifies on the side facing the Al layer, and at the end of the cooling process, an Al—Si eutectic solidifies between the highly aluminum doped layer and the silicon enriched layer. The highly aluminum-doped epitactically grown silicon layer is responsible for the passivation of the back surface of the solar cell. As a result of the high Al doping, a surplus of negatively charged, immovable Al acceptors is created, which produce an electric field that repels the minority carriers, the so-called back-surface field.
When the aluminum layer extends over the entire back surface of the solar cell and/or substrate, there is a soldering problem because it is not easy to directly solder tin-plated or non tin-plated metal connectors, for example, in particular copper connectors onto the aluminum back surface. In order to still carry out the required electric contacting, conductor paths having silver contacts or soldering points are directly applied onto the substrate surface by means of a screen printing, pad printing or other suitable printing process, and the tin-plated copper strips soldered onto them. Consequently, a recess in the aluminum layer is provided in the area of the soldering contacts resulting in that no back-surface field may form in this area so that the back surface of the solar cell is not completely passivated electrically and therefore less local photo currents appear. This has a negative impact on the degree of efficiency.
Since silver as a raw material is expensive, it should be dispensed with in order to reduce the manufacturing costs. Therefore, it is desirable to fully avoid the Ag contact.
Soldering the contact strips directly onto the aluminum layer is hardly possible for several reasons. One of the reasons is the oxidized surface of the Al particles. A further reason is that as a result of the sintering process, the upper aluminum surface is not sufficiently cohesive. Thus, during the sintering process an aluminum layer in the form of isolated spherical Al particles sintered together (sintered layer) originates over the silicon-doped alloy layer, where the aluminum bond is not a compact bond, but a rather loose sintered bond, which, depending on the composition of the aluminum paste and/or processing values during sintering is more or less porous. The pores may be filled with glass components.
If despite that, it should be possible to solder on this sintered aluminum layer, the hold would be very poor because of the porosity and consequent instability of the layer. This poor hold is noticeable as low pull-off forces of approx. 2-8 N, the sintered layer being torn apart so that the sphere structure of the particles may be recognized on both sides of the tear-off points. This entails the risk that the sintered layer may be destroyed on attempting to pull off the contact strips. The same occurs when the soldering connection on the aluminum layer is exposed to the pull-off forces actuating under operating conditions in a module. Small tears may result, which may lead to less durability of the soldering point and consequently also entail higher transfer resistance.